Refraction and lenses — GCSE Physics practice questions

Question 1

The laser in a supermarket barcode scanner produces monochromatic light.

What does monochromatic mean?

Accepted Answer

B: light that has a single frequency. Monochromatic light is light of a single frequency (one colour). A laser is a common source of monochromatic light.

Question 2

A pupil builds a model projector from a converging lens and a small illuminated slide.
The ray diagram shows how the lens forms an image of the slide.

Which row describes the image?

Accepted Answer

D: real · inverted · larger. The rays really meet at the image, so it is real and could be shown on a screen. The image arrow points down, so it is inverted, and it is taller than the slide, so it is larger (magnified). This is how a projector works: the slide is between F and 2F.

Question 3

In a model of a slide projector, a converging lens forms a sharp image of an illuminated arrow.
The diagram is drawn on a grid. The principal foci of the lens are not marked. 1 square on the grid represents 2.5 cm.

What is the focal length of the lens?

Accepted Answer

B: 10 cm. A ray from the object tip parallel to the axis meets the lens 2 squares above the axis and must pass through the image tip, 4 squares below the axis and 12 squares from the lens. This line crosses the axis 4 squares from the lens, at the principal focus. The focal length is 4 × 2.5 cm = 10 cm.

	real or virtual	upright or inverted	size compared with the slide
A	real	inverted	smaller
B	real	upright	larger
C	virtual	upright	larger
D	real	inverted	larger